1. Field of the Invention
The present invention relates to a device for driving, in a controlled manner, a sensorless brushless-DC-motor such as a motor used in an electric vehicle or a hybrid electric vehicle.
2. Description of Related Art
There are two types of brushless-DC-motors, one having a rotational position sensor and the other having no such sensor. A phase, frequency and rotational speed of the motor are determined based upon a rotor position detected by the rotational position sensor in the motor having the sensor, while those are determined based upon a rotor position estimated from a motor voltage imposed thereon in the motor having no sensor.
In a conventional sensorless brushless-DC-motor, a rotor position is determined based on a motor voltage detected, and a rotational speed of the motor Nm is determined from a cyclic period of a signal indicating the rotor position. The motor speed Nm is controlled to a target speed Nt by supplying a motor voltage that is PWM-controlled based on a difference between the motor speed Nm and the target speed Nt. More particularly, a duty ratio Dr of the pulse width modulated (PWM) voltage supplied to the motor is increased or decreased in accordance with the difference between Nm and Nt.
However, in the conventional control, there is a problem that the rotor position is erroneously determined when a DC power source voltage is abruptly changed, because the rotor position is determined based on a DC voltage detected. The erroneous determination of the rotor position may result in a halt of the motor due to a loss of synchronism, because the motor speed is determined from the rotor position, and the duty ratio of the PWM voltage is calculated based on the motor speed. The abrupt change of the DC power source voltage often occurs in an electric vehicle or a hybrid vehicle when it is operated under regenerative braking. In this case, the power source voltage abruptly increases due to the regenerated energy.
The problem above mentioned is especially notable under the following situation: A DC power source voltage of an electric vehicle is considerably low, and accordingly the motor is being driven at a speed lower than a target speed with a PWM duty ratio fixed to 100%. If the power source voltage abruptly increases, under this situation, due to regenerative braking, the duty ratio cannot quickly follow the abrupt increase of the power source voltage. This results in an abrupt increase of the motor torque, or a halt of a motor due to loss of synchronism caused by erroneous determination of the rotor position.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved motor controller which is able to stably control a sensorless brushless-DC-motor even if a power source voltage is abruptly changed.
A sensorless brushless-DC-motor mounted on an electric or hybrid vehicle is driven by an on-board DC power source, i.e., a battery. The DC power of the battery is inverted into a three-phase pulse width modulated voltage (a PWM voltage) through an inverter controlled by an associated controller. The PWM voltage is supplied to the motor to drive it in a controlled manner. An average level of the PWM voltage is controlled by changing a duty ratio of the PWM voltage. The duty ratio is controlled to minimize a difference between a target motor speed and an actual motor speed.
The actual motor speed is determined based on a signal indicating a rotor position which is in turn detected based on the PWM voltage imposed on the motor. On the other hand, the target motor speed is fed to the controller from outside. The PWM voltage is controlled so that fluctuation of the battery voltage does not influence the level of the PWM voltage. Since the rotor position is detected based on the PWM voltage thus controlled, it is detected with a high accuracy.
When the battery voltage drops for some reason and thereby the duty ratio reaches 100% to compensate for the battery voltage drop, the original target motor speed is reduced to a level of the detected actual motor speed. The reduced level is maintained until the duty ratio becomes lower than 100%. When the battery voltage increases again and the duty ratio becomes lower than 100%, the target motor speed is gradually increased to the original level. In this manner, when the battery voltage is abruptly increased by regenerative braking or the like, an excessive sudden increase of the PWM voltage is avoided. Accordingly, the rotor position is accurately detected based on the PWM voltage, and the motor is properly controlled without causing loss of synchronism.
According to the present invention, the sensorless brushless-DC-motor is stably controlled irrespective of an abrupt change of the battery voltage.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.